1. Field of the Invention
The present invention relates to a multi-layered electrophotographic positively charged organic photoconductor, and more particularly, a multi-layered electrophotographic positively charged organic photoconductor with superior electric properties, such as an improved sensitivity and a reduced discharge voltage in which a composition for forming a charge generating layer comprises a hole transport material, wherein charges may be readily injected to a charge transport layer.
2. Description of the Related Art
FIG. 1 illustrates a fundamental construction of a positively charged organic photoconductor used in the electrophotographic imaging process. As shown in FIG. 1, the multi-layered positively charged organic photoconductor has a structure comprising a charge transport layer (CTL) 20 and a charge generating layer (CGL) 30 sequentially coated on a conductive substrate 10. Since the charge generating layer 30 has a thin thickness, an overcoat layer (OCL) 40 may be coated thereon. Otherwise, the charge generating layer may be worn away by friction with a toner and a cleaning blade. Also, a charge blocking layer (not shown) may be provided to function as an adhesive and to block charges between the conductive substrate 10 and the charge transport layer 20.
The principle of forming an electrophotographic image using an organic photoconductor having the foregoing fundamental construction is as follows.
The surface of an organic photoconductor is positively charged by a charging device such as a corona. When a laser beam is irradiated on the charged organic photoconductor, positive charges (holes) and negative charges (electrons) are generated in the charge generating layer of the organic photoconductor. At this time, the positive charges are injected into the charge transport layer of the organic photoconductor by an electric field which has been previously applied to the organic photoconductor, and then migrate to the conductive substrate. On the other hand, the negative charges (electrons) migrate to the surface of the overcoat layer of the organic photoconductor to neutralize the surface charges. Thus, an exposed discharged part has its surface potential lowered to form a latent image, upon which a toner is developed to form an image on the surface of the organic photoconductor. The image thus obtained may be transferred to a receiver, such as paper or transferring products. The above electrophotographic process is repeated several times.
In the multi-layered positively charged organic photoconductor, the respective layers have roles different from each other, while a single-layered organic photoconductor satisfies a series of electric properties in a single layer. Therefore, it is much easier to design electric properties with respect to charge voltage and light exposure voltage with the multi-layered positive charged organic photoconductor. Also, an electric field may be stably applied to a multi-layered positive charged organic photoconductor in a thinly coated state, wherein the organic photoconductor may have a greater charge capacity. Thus, by using the multi-layered positively charged organic photoconductor, a wet toner having a large charge relative to a small particle size may be easily developed.
However, the multi-layered positively charged organic photoconductor has a disadvantage in that when a composition to form a charge generating layer is coated on a charge transport layer, an organic solvent in the composition may dissolve a part of the charge transport layer that is disposed under the charge generating layer. Consequently, the thickness of the charge transport layer may change, or substances making up the charge transport layer may be dissolved, which may cause a reduction in the charge transport capacity. Also, as the coating process is repeated, there are some problems including, for example, contamination of the composition that forms the charge generating layer.
To solve such problems, use of a solvent which does not dissolve any substance that forms the charge transport layer as an organic solvent in the composition to form a charge generating layer has been proposed. However, since such a method results in poor contact between charge generating material and charge transport material, charges generated in the charge generating layer by a laser beam cannot be injected to the charge transport layer effectively. Accordingly, the surface potential of an exposed part is not sufficiently low, and gradually increases when the photoconductor is repeatedly used.